Broadband wireless systems are in a rapidly evolutionary phase in terms of the development of various technologies, development of various applications, deployment of various services and generation of many important standards in the field. Although there are many factors to be considered in the design of these systems, the key factors have been the bandwidth utilization efficiency due to the limited bandwidth allocation, flexibility in operation and robustness of the communication link in the presence of various disturbances while achieving the specified performance. At present, the OFDM techniques have been adapted in many wireless communication standards, such as the World-wide Interoperability for Microwave ACCESS (Wimax), digital audio broadcasting (DAB), digital video broadcasting-terrestrial (DVB-T), Long Term Evolution (LTE), etc.
One of the advantages of the OFDM system is the mitigation of a major source of distortion present in high data rate wireless communication links, namely the inter symbol interference (ISI) achieved by reducing the symbol period by the use of multiple carrier transmission. However, the use of a large number of carriers based on the orthogonality property in the OFDM system makes the performance of the system very sensitive to any carrier frequency offsets introduced, for example, by the Doppler shifts encountered in the wireless channels. The proper operation of the OFDM system requires means for precise estimate of the Doppler that may be different for different carriers in the frequency selective fading channel, and means to mitigate such a Doppler effect from the received OFDM signal. Various methods exist in the prior art to solve this problem.
Another important problem arising with the use of a relatively large number N of carriers used in the OFDM signal is a relatively high peak to average power ratio resulting in a much reduced radio frequency (RF) power amplifier efficiency. Due to the inherent saturation in the RF power amplifier, the signal with amplitude exceeding the input linear range of the amplifier is clipped or distorted. In order to keep the distortion to some specified limit arrived at by the signal to distortion plus noise power ratio considerations, the output RF power is backed off from the maximum available power at the amplifier output and higher is the peak to average power ratio of the signal at the amplifier input, larger is the required back off in the output power. The output back off concurrently also results in the reduction of the DC to RF power conversion efficiency of the RF power amplifier thus increasing the drain on the battery or any other power supply source in the mobile devices. Another problem arising due to distortion caused by the amplifier is the spreading of the spectrum of the OFDM signal outside the allocated band.
Various methods exist in the prior art to solve the problem of high peak to average power ratio including the method taught by Kumar in, “Multi Transform OFDM Systems and Methods with Low Peak to Average Power Ratio Signals,” U.S. patent application Ser. No. 13/913,761. The simulation results with the Multi Transform OFDM Systems show that the system effectively eliminates any increase in the peak to average power ratio of the OFDM signal due to a relatively large number of carriers.
However, in the multiple accessing mode of the OFDM system wherein relatively narrow band OFDM channels are assigned to various users in a mobile communication network, the frequency selective fading of the communication channel may cause severe fading of some of the user signals resulting in frequent hand offs or call drops for such users in a mobile communication network. The solution to such a problem may be the transmission of the user signal over multiple OFDM channels for providing a diversity gain and/or transmission at a much reduced data rate. However, such a solution results in a reduced capacity of the OFDM system.
Another problem with the OFDM system is that the narrow band signals of various users have no protection against any intended or unintended interference. Any significant narrowband interference in any of the OFDM channels may disrupt communication to users assigned such channels.
Band limiting filters such as square root raised cosine filters are used in the OFDM systems to minimize the transmission bandwidth. Due to band limiting filtering effects a significant number of OFDM channels at the band edges are not assigned to any users as the edge channels are subjected to significant attenuation resulting in a significant reduction of the OFDM system capacity.
There is a strong motivation to come up with systems and methods that achieve higher bandwidth efficiency compared to the OFDM system by eliminating the edge channel effects of the OFDM system, and provide protection against deep fades in some segments of the spectrum and against narrowband interference.
The orthogonal frequency chirp multiple accessing (OFCM) system of the invention is a spread spectrum system wherein each user spectrum occupies the complete wide band of the OFCM system, thereby providing protection against both the interference and frequency selective fading. Deep fading in a few segments of the wide band spectrum may not result in any significant performance degradation to any of the users. The prior art frequency chirp method used in Radar and other systems is comprised of the transmission of a single frequency chirp signal. The OFCM system of the invention is for the transmission of multiple user signals over the same transmission band in a mutually non interfering manner.
The OFCM system of the invention requires approximately the same bandwidth as that for the OFDM system, however, it does not have any edge effects of the OFDM system wherein a significant part of the transmission band is not used for information transmission, as in the OFDM system the edge OFDM channels suffer from a disproportionate amount of attenuation due to the band limiting effects. In the OFCM system of the invention, each user's transmission is over the complete wide band of the OFCM system and any filtering effects may result in only a relatively minor degradation of performance for the users, thereby solving the problem of edge effects. In prior art the frequency chirp method used in the Radar and other systems is comprised of transmission of a single frequency chirp signal. The OFCM systems of the invention are for the transmission of multiple user signals in the same transmission band in a mutually non interfering manner.
In the OCFM system of the invention, individual user signals with modulations such as MPSK (Multi Frequency Shift Keying), are constant envelope signals inheriting the property from the frequency modulated (FM) signals with an advantage in terms of requiring relatively low amplifier back off in the user to base station transmission in a mobile communication network wherein the power efficiency is relatively more important. The OFCM system of the invention inherits advantages of both the OFDM system and the spread spectrum systems such as relatively high bandwidth efficiency of the OFDM system, and the protection against frequency selective fading and narrow band interference offered by the spread spectrum systems. These and other advantages of the OFCM system will be apparent from the detailed description of the invention.